1. Field of the Invention
The invention relates to a method for controlling the brightness and increasing uniformity of light generated by Lambertian surface sources, and more particularly to a method that controls the Lambertian surface sources to produce even light on a target surface.
2. Description of Related Art
Lighting devices are indispensable illuminating equipment in life and widely applied in either indoor or outdoor space, for example the street lights. Although the lighting devices have been developed for many years, the lighting device still can be improved in some aspects, such as energy converting efficiency between electricity and light. Conventional tungsten bulbs only convert approximate 5% of electrical energy to light energy, while other 95% of electrical energy turns to heat energy. In addition to the drawback of low energy converting efficiency, the generated heat energy generally results in heat-dissipating problems.
In view of the foregoing drawbacks of the conventional illuminating devices, LED-based lighting devices with features of low power consumption, long useful life are developed. However, the lighting angle of the LED-based lighting devices are limited to small degrees. Since the LED are designed to focus its light, the LED cannot be used in applications needing a spherical or wide light field.
With reference to FIG. 4, multiple Lambertian surface sources (70) are arranged in a line with equal intervals and radiates a target surface such as the ground. If the area (A) and flux (P) of each Lambertian surface source (70) are known, the brightness (N) can be expressed by
  N  =            P              π        ⁢                                  ⁢        A              .  
The illumination distribution is shown on FIG. 5. An angle α can be defined between a virtual normal of the Lambertian surface source (70) and a virtual line, where the virtual normal is perpendicular to the target surface at a point, and the virtual line extends from the edge of the Lambertian surface source (70) to the same point on the target. The angle α can be expressed by
      α    =                  tan                  -          1                    ⁡              (                  X                      2            ⁢                                                  ⁢            L                          )              ,wherein X is a diameter of each Lambertian surface source (70) and L is the distance from the target surface to the Lambertian surface source (70), i.e. the length of an opposite side of the right triangle with the internal angle α.
With reference to FIG. 6, a center illumination (H0) on the target surface and other illumination values (Hθ) along other included angles (θ) on the target surface can be calculated.H0=πN sin2 αHθ=H0 cos4 θ
The center illumination (H0) has the maximum value than other illumination values (Hθ) along other included angles (θ). Still referring to FIG. 5, in order to generate even illumination on the target surface, a portion of lighting area provided by one Lambertian surface source (70) overlaps a portion of lighting area provided by an adjacent Lambertian surface source (70). Although the light emitting from different Lambertian surface source (70) may radiate the same region to minimize the difference between the illumination value (Hθ) and the center illumination (H0), the illumination intensity on the overlapped region is still insufficient.
To overcome the shortcomings, the present invention provides a method for controlling brightness and increasing uniformity of light generated by Lambertian surface sources to mitigate or obviate the aforementioned problems.